In the art of optical fiber installation and maintenance, several instruments are currently in use requiring the measurement of relatively weak optical signals in the field. For example, a handheld system is presently in use for pinpointing particular optical fibers containing live traffic. The fibers are identified in a non-intrusive manner, without the necessity to cut or terminate the fibers. That fiber identification system is disclosed in U.S. Pat. No. 7,283,688 to Frigo, Iannone and Reichmann, issued Oct. 18, 2007, the contents of which is hereby incorporated in its entirety herein. A system according to that disclosure is marketed by EXFO Electro-Optical Engineering Inc. of Quebec City, Quebec, Canada under the name FiberFinder™.
The device includes two units: a tone generator and a live fiber detector. The tone generator imparts a time-varying modulation onto an optical signal propagating in the optical fiber by vibrating an actuator to modulate the curvature of a bend of the fiber, thereby generating a time-varying loss of power in the signal. For example, a piezoelectric transducer may be used to impart the signal.
An example live fiber detector 100 is shown in FIG. 1. An optical fiber 110 is bent around a clamping anvil 131, creating a bend in the fiber so that a portion of the optical signal carried by the fiber is scattered out of the fiber into a light guide 151. The scattered light 155 is guided by the lightguide 151 into a detector 121, which converts the received light into an electrical signal that is analyzed to identify the signal and identify the optical fiber 110.
The scattered light 155 is relatively weak and the detector 121 must therefore be sensitive. A cover or cap 140 is used in the field to shield the detector from ambient light that would otherwise compromise the measurement. The cover must allow the fiber 110 to enter the darkened interior, and for that purpose openings 145a, 145b are provided.
The openings 145a, 145b, however, have been found to admit some ambient light, represented in FIG. 1 as arrows 171. That spurious admitted light has been found to significantly degrade the performance of the device 100 by interfering with the measurement of the scattered light 155 from the fiber 110, disrupting the fiber identification process.
The problem is aggravated by the necessity of providing openings 145a, 145b large enough to accommodate the largest optical fiber expected to be identified using the device. The large openings create a large gap around the fiber when a smaller fiber is measured. For example, a device might have an opening large enough for a fiber having a 3 mm outer jacket, but might be used to identify a 900 μm fiber, leaving a gap of over 1 mm around the circumference of the fiber.
Additionally, because the system is designed to operate in an in-line manner on the fiber, the openings 145a, 145b must in practice be slots having open ends for placing over the optical fiber. Longitudinal slots also permit movement of the fiber during the bending process. As compared to round openings, the slots allow additional ambient light to enter the cover.
One approach to solving the problem of ambient light has been to offset the scattered light reading based on an initial ambient light reading. While that technique is effective under certain conditions, it is not completely effective under intense ambient light conditions and under changing ambient light conditions.
There is therefore a need for a low-cost and effective apparatus and method for reducing or eliminating spurious light entering the interior of an optical fiber identification device and interfering with fiber identification. To the inventors' knowledge, no such methods or apparatus currently exist.